1. Field of Invention
This invention relates to a sensor for detecting shocks (hereinafter referred to as "shock sensor"), in particular, this shock sensor detects varying accelerations of vehicles, such as motor cars, in, for example, the situations of car collisions, sudden stops, sudden starts or sudden accelerations, and detects car collisions and other vehicle situations. In particular, this shock sensor has a magnetic reed switch, which detects shocks that vehicles receive at the time of vehicle collisions and other traffic accidents, and outputs the signal to actuate a protective device, such as an air bag or seat belt system.
2. Prior Art
In the prior art, there are many examples of shock sensors using a magnetic reed switch. These shock sensors have a housing mounted within the car, wherein the switch case is placed in the direction in which a shock is to be detected, for example, in the direction in which the motor car moves. In the switch case, the magnetic reed switch is held, and a magnet of cylindrical shape is held and placed coaxially with the switch case. The magnet is biased by a coil spring so that it may slide along the switch case.
When a motor car stops suddenly due to a car collision or other cause, the shock sensor receives the rather significant deceleration, and as a result, the magnet in the shock sensor, based on its inertial mass, under the inertial force, moves along the switch case against the resistance of the bias of the coil spring.
When the magnetic force applied to the contact portion of the magnetic reed switch exceeds a predetermined figure, the contact portions are closed causing the switch to be in the on state Afterward, as the motor car stops, the tension of the spring becomes greater than the inertial force of the magnet, and the magnet moves to the opposite direction along the outer surface of the switch case, back to its original position prior to the collision. At this time, the magnetic force applied to the magnetic reed switch gradually decreases, and the contact portions of the switch are converted to the off state, when the magnetic force decreases to a level below the predetermined figure.
The magnetic reed switch remains in the on state from the time the contact portions are closed until the time when the contact portions are opened. The on state of the contact portions is detected by a predetermined detecting circuit, and the signal from the detecting circuit is entered into the controller for the passenger protection safety system, such as air bags.
However, when the shock applied to the conventional shock sensor is extremely great, the speed of the magnet moving along the outer surface of the switch case increases, and, as a result, the on state of the magnetic reed switch ends after a very short time, and it is possible that the shock detecting signal from the detecting circuit would also be output for a very short time.
Moreover, in a conventional shock sensor, the distortion of the coil spring in the direction of its diameter is restricted such that it only occurs when the coil spring is compressed very strongly, causing a problem in that the period during which the magnetic reed switch continues to be in the on state varies, case by case.
When assembling the conventional shock sensor, the coil spring may be put between the housing and the switch case, and resin used to seal the reed switch may adhere to the coil spring placed between the housing and the switch case. This may prevent expansion and contraction movements of the coil spring, and thus may cause malfunctions of the shock detecting system.